62 LILLIE. [Vol. X. 



dermic sac.^ During the metamorphosis the first two pairs 

 move backwards with the thread-gland nearly to the oral plate. 

 They thus come to be associated with the third pair of sensory 

 hairs, which lie near the oral plate. When the invagination of 

 the larval mantle takes place it is these three tufts which lie 

 beneath the shell-hooks on each side. By this time the oral 

 and ventral plates have grown together, and the fourth pair of 

 sensory hairs now lies a little in front of the oral plate, and to 

 the sides of the aperture of the thread-gland. 



From the posterior angle of the valves of the shell to the 

 oral plate — which is now assuming the character of a stoma- 

 deal invagination (in February glochidia of Anodonta) — the 

 whole median surface is formed by cells of the ventral plate. 

 The further growth forward of this plate is what causes the 

 anterior displacement, in parasitic glochidia, of the oral plate 

 to its definitive position. The foot — which is formed in this 

 region — is then derived from cells of the ventral plate. It of 

 course goes without saying that during these shiftings the 

 larval mantle has invaginated. Having thus outlined the meta- 

 morphosis, it now remains to treat of each stage in detail. 



The most striking difference between the young larva and 

 the glochidium is the bifid condition of the mantle in the latter 

 as contrasted with its unpaired condition in the former. It has 

 been known since the time of Flemming's paper that the 

 difference is established by the invagination towards the dorsal 

 line of the whole ventral surface of the young larva along the 

 median plane. But while this has in general been known, 

 the histological changes which must accompany such a stu- 

 pendous transformation have never been described. These are 

 illustrated in Figs. 83 to 91. The preparation consists, first, 

 in the establishment of a line of suture-cells (Figs. 80, 82, and 

 86) which divides the basis of the larval mantle into two halves, 

 from the thread-gland to the oral plate; and, second, in the 

 vacuolation of the cells which are to invaginate. Invagination 



^ This differs from Schierholz's description of their original position. It may 

 be, however, that he overlooked their earliest appearance, when they are seen 

 with difficulty. Schierholz described three pairs as lying near together, just 

 beneath the edge of the shell, at the transverse level of the oral plate. This is 

 not their original position in Unio. 



